The present invention relates to a heat-resistant cast steel suitable for exhaust equipment members for automobiles, etc., and an exhaust equipment member made of such a heat-resistant, austenitic cast steel. More specifically, it relates to a heat-resistant, austenitic cast steel with excellent castability and machinability and further excellent strength at 900.degree. C. or higher and an exhaust equipment member made of such a heat-resistant, austenitic cast steel.
Some of conventional heat-resistant cast iron and heat-resistant cast steel have compositions shown in Table 1 as Comparative Examples. In exhaust equipment members such as exhaust manifolds, turbine housings, etc. for automobiles, heat-resistant cast iron such as NI-RESIST cast iron (Ni-Cr-Cu austenitic cast iron) and heat-resistant cast steel such as ferritic cast steel shown in Table 1 are employed because their operating conditions are extremely severe at high temperatures.
Further, attempts have been made to propose various types of heat-resistant, austenitic cast steel. For instance, Japanese Patent Laid-Open No. 61-87852 discloses a heat-resistant, austenitic cast steel consisting essentially of C, Si, Mn, N, Ni, Cr, V, Nb, Ti, B, W and Fe showing improved creep strength and yield strength. Japanese Patent Laid-Open No. 61-177352 discloses a heat-resistant, austenitic cast steel consisting essentially of C, Si, Mn, Cr, Ni, Al, Ti, B, Nb and Fe having improved high-temperature and room-temperature properties by choosing particular oxygen content and index of cleanliness of steel. Japanese Patent Publication No. 57-8183 discloses a heat-resistant, austenitic cast Fe-Ni-Cr steel having increased carbon content and containing Nb and Co, thereby showing improved high-temperature strength without suffering from a decrease in high-temperature oxidation resistance. Further, Japanese Patent Laid-Open No. 5-5161 discloses a heat-resistant, austenitic cast steel of Fe-Ni-Cr containing Nb, W, Mo, B and Co for drastically improving high-temperature strength.
Among these types of conventional heat-resistant cast iron and heat-resistant cast steel, for instance, the NI-RESIST cast iron is relatively good in a high-temperature strength at up to 900.degree. C., but it is poor in durability at 900.degree. C. or higher and expensive because of a high Ni content. On the other hand, the heat-resistant, ferritic cast steel is extremely poor in a high-temperature strength at 900.degree. C. or higher.
Since the heat-resistant, austenitic cast steel disclosed in Japanese Patent Laid-Open No. 61-87852 has a relatively low C content of 0.15 weight % or less, it shows an insufficient high-temperature strength at 900.degree. C. or higher. In addition, since it contains 0.002-0.5 weight % of Ti, harmful non-metallic inclusions may be formed by melting in the atmosphere.
In addition, since the heat-resistant, austenitic cast steel disclosed in Japanese Patent Laid-Open No. 61-177352 contains a large amount of Ni, it may suffer from cracks when used in an atmosphere containing sulfur (S) at a high temperature.
Further, since the heat-resistant, austenitic cast steel disclosed in Japanese Patent Publication No. 57-8183 has a high carbon (C) content, it may become brittle when operated at a high temperature for a long period of time.
The heat-resistant, austenitic cast steel disclosed in Japanese Patent Laid-Open No. 5-5161 is suitable for exhaust equipment members exposed to high temperatures, but it is inherently poor in castability and machinability as austenitic cast steel.